leetcode Question 101: Spiral Matrix II

Spiral Matrix II

Given an integer n, generate a square matrix filled with elements from 1 to n² in spiral order.

For example,
Given n = 3,

You should return the following matrix:

[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

Analysis:
Having solved the previous problem. This problem seems easier than the last one.

The key idea is to consider the single step ----- "Which direction to go filling the next number?"
There are only four directions.
Be careful with one thing----- you keep going one direction until meet the end.


Code(Updated 2013.10):

class Solution {
public:
    vector<vector<int> > generateMatrix(int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<vector<int> > res(n,vector<int>(n,0));
        if (n==0){return res;}
        int i=1;
        int x = 0;
        int y = 0;
        res[0][0]=i++;
        while (i<=n*n){
            while (y+1<n && res[x][y+1]==0){   // keep going right
                res[x][++y]=i++;
            }
            while (x+1<n && res[x+1][y]==0){   // keep going down
                res[++x][y]=i++;
            }
            while (y-1>=0 && res[x][y-1]==0){  // keep going left
                res[x][--y]=i++;
            }
            while (x-1>=0 && res[x-1][y]==0){  // keep going up
                res[--x][y]=i++;
            }
        }
        return res;
    }
};